Cardiac fibroblasts are the most numerous cells within the heart. Their traditional roles are the maintenance of the extracellular matrix to support the structure and contraction of the heart, and their activation and production of increased extracellular matrix in disease. Over the past 15 years, evidence has grown that shows fibroblasts are capable of modulating myocyte function. This is achieved through paracrine mediators, the release of biologically active soluble substances into the local environment, and through direct cell contact, involving gap junctions and mechanical connections. Fibroblasts can also modulate myocyte function indirectly through modification of the extracellular matrix which can transfer various signals to myocytes. However, research into the interaction between fibroblasts and myocytes has been largely limited to using neonatal cells and fibroblasts that have been maintained in culture.
This thesis sets out to examine the paracrine effects of fibroblasts on myocyte structure and excitation contraction coupling. Adult myocytes and, where possible, fibroblasts before their activation in culture were used. The thesis also aimed to determine whether these effects are different using fibroblasts after pressure overload of the myocardium and potential mediators involved in the effects.
Initial studies looked at the effect of normal rat cardiac fibroblasts isolated from healthy rat hearts, and showed that these fibroblasts reduced myocyte viability, induced myocyte hypertrophy and increased the amplitude of the Ca2+ transient. We then compared the effect of fibroblasts from pressure overloaded hearts with control fibroblasts. The pressure overloaded fibroblasts caused a similar reduction in myocyte viability and also induced myocyte hypertrophy. However, the functional effects were different and the Ca2+ transient amplitude was reduced in myocytes co-cultured with pressure overload fibroblasts.
TGF-β was elevated in the both pressure overload and control fibroblasts co-cultures and was therefore investigated as the potential mediator of the effects. It was found that fibroblast derived TGF-β directly causes myocyte hypertrophy, whereas the other effects are due to more complex signalling with the involvement of secondary mediators released from the fibroblasts in response to TGF-β signalling.
Finally, we investigated the paracrine effects of cultured canine fibroblasts on the electrical activity of adult myocardium using canine myocardial slices. These are thin viable slices of the left ventricle and allowed us to investigate the field potential and the conduction velocity of intact adult myocardium. It was found that fibroblasts-derived paracrine mediators altered the conduction velocity of these slices.
This work supports the growing body of evidence that fibroblasts are capable of modulating myocyte structure and function and, specifically, myocyte excitation contraction coupling. We have shown that these effects are evident in adult cells and are altered using fibroblasts from normal and diseased heart. Furthermore, we have shown that TGF-β appears to be central to these effects.